True vapor lock is about as rare as finding the Easter Bunny, a chicken with lips, or an honest politician. You've probably got a bad inlet check valve in your mechanical fuel pump. Quite often, it's caused by debris from the fuel tank getting lodged in the valve and preventing it from working properly. There's a test to diagnose that problem that I've been describing for 20+ years on the forum, but it's one that few, if any bolters appear to be willing to do.
Jerry
